Devices using semiconductors, such as computers (CPUs), transistors, and light-emitting diodes (LEDs), generate heat when used, and the performance of electronic parts may be degraded due to the heat. Therefore, radiators are attached to such electronic parts that generate heat. However, such radiators are usually made of metal, and the radiating portion does not adhere well to electronic parts. Accordingly, a method that interposes a thermally conductive composition processed in a sheet form therebetween to enhance adhesion has been employed. However, the recent advances in the performance of electronic parts is significant, and accordingly the amount of heat generated has become very large. Therefore, research efforts actively have been made on thermally conductive polymer compositions having enhanced thermal conductivity. Such thermally conductive polymer compositions have to contain large amounts of thermally conductive inorganic powder for the enhancement of the thermal conductivity of heat-dissipating materials, which is the ultimate goal. It is, however, known that a mere increase in the amount of thermally conductive inorganic powder results in various problems. For example, there are problems in that the hardness is increased excessively in the case of an elastomeric heat-dissipating material, thereby not allowing a specifically narrow space to be provided between an electronic part and a radiator, or the gap between an electronic part and a radiator to be filled as desired. Moreover, in the case of an elastomeric or gel heat-dissipating material, compression set is increased and long-term reliability is likely to be deteriorated. Furthermore, there are problems in that, for example, the viscosity of a composition prior to curing is increased, thereby greatly impairing the workability, or the change over time of curing characteristics is aggravated.
To address these problems, various methods have been proposed. Methods that use a thermally conductive inorganic powder that has a specific particle size distribution or shape, or combinations of several types of thermally conductive inorganic powders have been proposed. Previously proposed are the use of a thermally conductive inorganic powder having a broad particle size distribution (Patent Document 1), a heat-dissipating material that uses 10 to 50 μm of spherical alumina and less than 10 μm of nonspherical alumina (Patent Document 2), the use of 0.1 to 5 μm of amorphous alumina and 5 to 50 μm of spherical alumina (Patent Document 3), the use of alumina having an average particle diameter of 2 to 10 μm and an oil absorbency of 15 ml/g (Patent Document 4), etc. Furthermore, methods in which the surface of a thermally conductive inorganic powder is treated also have been proposed, and there are proposals of a heat-dissipating material in which a surface treatment agent is applied to a combination of zinc oxide and magnesium oxide (Patent Document 5), a treatment with a long-chain aliphatic alkylalkoxysilane having 6 or more carbon atoms (Patent Document 6), a treatment with siloxane having an alkoxysilyl functional group at one terminal (Patent Document 7), and a treatment of a thermally conductive inorganic powder with a silane coupling agent (Patent Document 8). While these conventional methods attain high thermal conductivity and excellent heat dissipation, they result in extensive outgassing due to the surface treatment agent and in increased rubber hardness, and are highly problematic in terms of the storage stability of the materials.